In modern semiconductor devices, the ever increasing device density and decreasing device dimensions demand more stringent requirements in the packaging or interconnecting techniques of such devices. Conventionally, a flip-chip attachment method has been broadly used in the packaging of IC chips. In the flip-chip attachment method, instead of attaching an IC die to a lead frame in a package, an array of solder balls is formed on the surface of the die. The formation of the solder balls is normally carried out by an evaporation method of lead and tin through a mask for producing the desired alloy balls. More recently, the technique of electro-deposition has been used to produce the solder balls in flip-chip packaging.
Other solder ball formation techniques that are capable of solder-bumping a variety of substrates have also been proposed. These techniques work fairly well in bumping semiconductor substrates that contain solder structures over a minimum size. One of the more popularly used techniques is a solder paste screening technique which can be used to cover the entire area of an 8 inch wafer. However, with the recent trend in the miniaturization of device dimensions and the reduction in bump-to-bump spacing (or pitch), the solder paste screening technique has become impractical. For instance, one of the problems in applying solder paste screening technique to modern IC devices is the paste composition itself. A paste is generally composed of a flux and solder alloy particles. The consistency and uniformity of the solder paste composition therefore become more difficult to control with a decreasing solder bump volume. A possible solution for this problem is the utilization of solder pastes that contain extremely small and uniform solder particles. However, this can only be done at a high cost penalty. Another problem in using the solder paste screening technique in modern high density devices is the reduced pitch between bumps. Since there is a large reduction in volume from a screened paste to the resulting solder bump, the screen holes must be significantly larger in diameter than the final bumps. Thus stringent dimensional control of the bumps makes the solder paste screening technique impractical for applications in high density devices.
A more recently developed injection molded solder (IMS) technique attempted to solve these problems by dispensing molten solder instead of solder paste. However, problems have been observed when the technique is implemented to wafer-sized substrates, U.S. Pat. No. 5,244,143, assigned to the common assignee of the present invention, discloses the injection molded solder technique and is hereby incorporated by reference in its entirety. One of the advantages of the injection molded solder technique is that there is very little volume change between the molten solder and the resulting solder bump. The IMS technique teaches the use of a two inch wide head that fills boro-silicate glass molds that are wide enough to cover most single chip modules. A narrow wiper provided behind the solder slot passes the filled holes once to remove excess solder. However, when a two inch wide head is used to fill molds for large wafers, i.e., such as an 8 inch or 12 inch wafer, the fill requires at least four or six successive scans by the head. During such successive scans, the overlapped areas between scans inevitably have degraded fill characteristics such as solder streaks between holes and non-uniform fills.
Another disadvantage of the IMS technique is the mold flatness and the head flatness. The boro-silicate glass molds used are typically thin enough to allow some flexibility over a length of 8 or 10 inches. At a typical thickness of 1/16", the large scale flexibility of the mold causes the mold to conform to the contour of the support-plate that holds the mold. During a relatively fast heating and cooling of the support-plate in the solder-fill process, the support-plate deforms over a large wafer-sized area. Under the IMS head pressure, the mold conforms to the support-plate contour and therefore becomes curved over its entire width. When compressed by a rigid IMS head, there is a high likelihood that a gap will be formed between the head and the mold. The gap causes poor wiping of excess solder from the mold surface resulting streaking and poor filling problems. Furthermore, the IMS technique requires vacuum to induce a solder flow by generating a negative pressure at the leading edge of the solder slot. The molten solder will leak into the vacuum slot when the gap between the vacuum and the solder slot is larger than a maximum allowable value, typically 5 .mu.m. Moreover, when both the mold and the head are made of glass material, the friction generated by glass sliding on glass causes a significant drag on the scanning head. Any hard debris on the mold surface may also cause significant damage to the mold.
It is therefore an object of the present invention to provide a method for forming solder bumps by a molten solder screening technique that does not have the drawbacks and shortcomings of the conventional solder bumping techniques.
It is another object of the present invention to provide a method for forming solder bumps by a molten solder screening technique that does not require the use of a vacuum source and a vacuum slot in the mold head.
It is a further object of the present invention to provide a method for forming solder bumps by a molten solder screening technique wherein a molten solder die of sufficient length to cover the entire area of a large wafer is used.
It is another further object of the present invention to provide a method for forming solder bumps by a molten solder screening technique in which a flexible die head capable of conforming to an uneven mold surface is used.
It is still another object of the present invention to provide a method for forming solder bumps by a molten solder screening technique in which fresh, un-oxidized molten solder is used for each mold fill.
It is yet another object of the present invention to provide a method for forming solder bumps by a molten solder screening technique in which a pressure means is used in combination with a flexible die to accommodate glass molds with large curvatures.
It is still another further object of the present invention to provide an apparatus for forming solder bumps by a molten solder screening technique in which a mechanical support means is used for engaging a mold with a flexible die such that a predetermined pressure is maintained between the mold cavities and the surface of a molten solder stream.
It is yet another further object of the present invention to provide an apparatus for forming solder bumps by a molten solder screening technique wherein an excess solder removal means is used to remove excess molten solder from the surface of the mold.
It is still another further object of the present invention to provide an apparatus for forming solder bumps by a molten solder screening technique in which a mold constructed of a material that has a coefficient of thermal expansion substantially similar to that of silicon or the final solder receiving material is used.